Connected: An Internet Encyclopedia
RFC 792 - Redirect Message

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### Redirect Message

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |     Code      |          Checksum             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Gateway Internet Address                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Internet Header + 64 bits of Original Data Datagram      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       IP Fields:

       Destination Address

          The source network and address of the original datagram's data.

       ICMP Fields:

       Type

          5

       Code

          0 = Redirect datagrams for the Network.

          1 = Redirect datagrams for the Host.

          2 = Redirect datagrams for the Type of Service and Network.

          3 = Redirect datagrams for the Type of Service and Host.

       Checksum

          The checksum is the 16-bit ones's complement of the one's
          complement sum of the ICMP message starting with the ICMP Type.
          For computing the checksum , the checksum field should be zero.
          This checksum may be replaced in the future.

       Gateway Internet Address

          Address of the gateway to which traffic for the network specified
          in the internet destination network field of the original
          datagram's data should be sent.

       Internet Header + 64 bits of Data Datagram

          The internet header plus the first 64 bits of the original
          datagram's data.  This data is used by the host to match the
          message to the appropriate process.  If a higher level protocol
          uses port numbers, they are assumed to be in the first 64 data
          bits of the original datagram's data.

       Description

          The gateway sends a redirect message to a host in the following
          situation.  A gateway, G1, receives an internet datagram from a
          host on a network to which the gateway is attached.  The gateway,
          G1, checks its routing table and obtains the address of the next
          gateway, G2, on the route to the datagram's internet destination
          network, X.  If G2 and the host identified by the internet source
          address of the datagram are on the same network, a redirect
          message is sent to the host.  The redirect message advises the
          host to send its traffic for network X directly to gateway G2 as
          this is a shorter path to the destination.  The gateway forwards
          the original datagram's data to its internet destination.

          For datagrams with the IP source route options and the gateway
          address in the destination address field, a redirect message is
          not sent even if there is a better route to the ultimate
          destination than the next address in the source route.

          Codes 0, 1, 2, and 3 may be received from a gateway.

Next: Echo or Echo Reply Message

Connected: An Internet Encyclopedia
RFC 792 - Redirect Message